Meters capable of determining the clarity or opacity of fluids are highly useful for monitoring fluids such as air, water or process fluids, such as lubricating oil. Various electronic instruments have been proposed for this service, such as described in U.S. Pat. No. 3,617,756, but they are relatively complicated and expensive and subject to malfunctions when operated in a contaminating environment. An object of this invention is to provide a relatively simple, economical and efficient optical device for monitoring the clarity or opacity of the fluid and it more particularly relates to such a device for use as a smoke detector on a smokestack.